A Mealy Machine is a type of finite state machine, where the outputs depends not only on the current state, but also upon the current inputs. Thus, it typically exhibits the following behavior:

• The output may change asynchronously, which can result in little "blips" of undesired output on a timing diagram. (Especially if the machine is an asynchronous sequential network.)
• The change in output often takes place up to one clock cycle before it would in an equivalent Moore machine.

See also the diagram in 'finite state machine' for a block diagram of a Mealy Machine.

This is sometimes called a Class A Machine.

Oppose: Moore Machine, Class B Machine, Class C Machine.

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Note: I've got some sources that say this is supposed to be spelt Mealey Machine, and some that say Mealy Machine. Anyone with a better knowledge of digital systems history care to comment?

Update: I decided to do a search on Google, and it returned 300 hits for "Mealy machine" and only 7 hits for "Mealey machine".